Method of preparing propellants using multimodal grains of beta-octogen

ABSTRACT

A method of preparing propellants involves using β-octogen with a multimodal grain size distribution and an average grain size of less than 50 μm in a propellant composition containing other explosives, inert binders and active binders.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method of using beta-octogen having a multimodal grain size distribution with an average grain diameter of less than 50 μm in the preparation of propellants and to the resulting propellants.

[0002] The term “octogen” as employed in the present invention is understood to mean, as is known, 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane or also cyclotetramethylene-tetranitramine. This compound is also known under the name of “HMX” (high melting explosive of His Majesty's explosive). Four crystalline modifications are known of this compound, namely the orthorhombic alpha form, the monoclinic beta form, the monoclinic gamma form, and the hexagonally crystallizing delta form. The beta form is a customary modification, the quality control of which is described in Military Specification MIL-H-45444 B (PA), Amendment 1, Jul. 15, 1975. A distinction is made between purity grades A and B. Moreover, the grain sizes are differentiated in accordance with several classes.

[0003] Heretofore, beta-octogen has been utilized in the state of the art as the sole explosive in passivated form, as a booster charge, in mixtures with TNT (octols) and others, and also as an explosive in solid rocket propellants or in barrel-type weapon propellants. The conventional areas of usage thus cover detonative as well as explosive reactions or deflagrations.

[0004] The term “deflagration” is understood to mean, as is known, reactions occurring substantially below the speed of sound in the material. The reactions are propagated by the thus-released heat of reaction. The reaction products flow away in opposition to the propagation direction. In contrast thereto, the detonative reaction of an explosive, as is known, is coupled with a shock wave during its reaction.

[0005] For use of an explosive as a propellant in barrel-type weapons, especially in hand firearms, it must be possible to preclude a detonative reaction:

[0006] The burning of a propellant powder is a deflagration process. The determination of the deflagration velocity can here be utilized, inter alia, as a suitable measuring variable. The linear burning rate of a propellant is the velocity at which the chemical reaction progresses from the ignition site. This rate is dependent on the composition, the pressure, the temperature, the physical state of the propellant, and the shape of the propellant.

[0007] The physical state of the propellant along these lines includes, in particular, the porosity, compactness, and grain size and grain size distribution of the components. In the use of beta-octogen as the energy carrier of propellants, such dependencies have been described in the literature. In Technical Memorandum 33-801, entitled “Nitramine Propellant Research” of the Jet Propulsion Laboratory, Pasadena, Calif., Oct. 15, 1976, the internal ballistics of fine-grained beta-octogen is examined. The following report is rendered on the dependency of the linear burning rate from the muzzle pressure:

[0008] “The use of a fine grain size should be an effective step for shifting the occurrence of breaking points, ‘recognizable by a sudden, strong increase in muzzle pressures’, in nitramine propellants. However, the actual performance of this step necessitates limitation of propellants with respect to their fineness. If the propellant contains only 5 μm octogen, the breaking point does not appear below about 2400 bar (35,000 psi). However, in reality, octogen charges will exhibit size distributions. For example, it has been reported of a charge of octogen with a major diameter of 5 μm that this charge contained particles from the sub-micron range up to 50 μm unless a careful screening step was first performed. Such a size distribution is obtained, in a model, by a tetramodal distribution of unequal quantities of 1 μm, 5 μm, 10 μm and 20 μm octogen.

[0009] “The breaking point of the 20 μm particles occurs at 280 bar (4,000 psi), the breaking point of the 10 μm particles at 420 bar (6,000 psi), and that of 5 μm particles at 1380 bar (20,000 psi). The effects of the 10 μm and 20 μm particles are greater than could be expected from their low concentration (in total 17%), on account of their deep penetration during combustion on the surface structure of the propellant. For a smooth (continuous) size distribution, an actual propellant would exhibit a prolonged transition in place of a stepped transition. As a consequence, when following corresponding methods, it is necessary to screen out coarse particles in the batches in order to obtain fine octogen. This is of greater importance in case of propellants containing inert binders than in case of propellants containing active binders, due to the moderation of propellant combustion by active binders.”

[0010] According to this description, an undesirable jump when considering the curve of the breaking points appears as early as a grain size of 5 μm, so that it can be seen therefrom that beta-octogen having such a small grain size is unsuitable for use in propellants. Inasmuch as even in tetramodal distribution such breaking points likewise occur already at relatively low pressures, the use of beta-octogen in barrel-type weapon propellants appears to be excluded.

[0011] The manufacture of particles having a narrowly limited, practically unimodal grain size distribution is extraordinarily expensive and can only be achieved by complicated screening operations. Although the memorandum reveals that the occurrence of the breaking points can apparently be mitigated by the presence of an active binder, useful results are not obtained in this case, either.

[0012] The pressure exponent can be determined from the thus-measured curves with the aid of an ascending slope. For propellants for barrel-type weapons, an excessively steep pressure rise is undesirable, as set out above. It is known from investigations of the German-French Research Institute, St. Louis, that the following applies for nitramine powders based on hexogen: The finer the grain, the lower the pressure exponent of the deflagration velocity, and the deflagration velocity itself. In this connection, a pressure exponent of <1 is obtained with hexogen grain sizes of 4-15 μm. In spite of a reduction in the pressure rise gradient, however, the maximal pressures are too high, at more than 6,000 bar. (Report S-R 906/83, H. H. Licht, A. Baumann, St. Louis, Apr. 13, 1983, page 11.)

[0013] DE 3,614,173 C1 discloses a granulated beta-octogen having a grain size of less thai 50 μm which is encased by synthetic resins.

[0014] U.S. Pat. No. 3,959,042 describes the use of beta-octogen (HMX) in propellant charges, introduced into a solution of an inactive binder.

[0015] DE 2,753,555 C1 describes, inter alia, the use of beta-octogen in conjunction with high proportions of active binders, accompanied by high proportions of inactive binders and fillers. The disclosure contains no data on the grain size or the grain size distribution of the beta-octogen utilized.

[0016] DE 3,617,408 C1 discloses a process for the production of fine-grained beta-octogen.

SUMMARY OF THE INVENTION

[0017] The object of the present invention resides in preparing an octogen having a suitable grain configuration, grain size, and grain size distribution for the prevention of jumps in the curves of linear burning rate versus pressure. Moreover, the pressure exponent of the deflagration rate should be <1. Furthermore, the present invention has as its objective a reduction of the pressure rise gradient and of the maximum pressure in the cartridge chamber as compared with the state of the art.

[0018] The aforementioned object has been attained by the use of beta-octogen having a multimodal grain size distribution with an average grain diameter of less than 50 μm in propellants made up of explosives, inert binders, and active binders.

DETAILED DESCRIPTION OF THE INVENTION

[0019] A multimodal grain size distribution along the lines of the present invention means a grain size spectrum as can be illustrated, in particular, by a Gaussian distribution curve. The production of especially fine-grained beta-octogen has been known from DE 3,617,408 C1. According to this document, a solution of beta-octogen in a gamma-lactone is treated with toluene in a temperature range of between 5° and 15° C.; the desired fine and very fine beta-octogen crystals being precipitated in high purity. In order to prevent grain growth, the very fine beta-octogen, after separation from the toluene, can be made into a slurry with water and combined, at temperatures of between 30° and 60° C., with a solution, suspension or emulsion of a polymer under agitation. During this step, the product is encased by the polymer.

[0020] Another process for producing the very fine beta-octogen grain resides in comminuting a commercially available product, as classified according to MIL-H-45444 B. In this case, the grain sizes range from about 45 μm up to several hundred μm. However, comminution of the crystals by grinding poses great difficulties since beta-octogen is extremely sensitive to friction and shock. Therefore, this procedure can only be performed under special safety precautions in appropriate devices.

[0021] Another process for the production of an especially fine-grained beta-octogen resides in the separation of the desired grain fractions. The starting material can be derived in this method from normal production or from the previously described comminution procedure. The separating methods are known per se. The use of hydrocyclones is of advantage in the production of grains according to the present invention. It is self-evident that an especially fine grain constitutes merely a fraction of the bulk employed, whereby the process Per se becomes more expensive. The special advantage of the direct production of fine grains resides in that the crystals from the crystallization are intact, as contrasted to those from the comminution process.

[0022] When using such a beta-octogen in propellants for hand firearms, it has now been found surprisingly that

[0023] no jumps occur in the curve of linear burning rate versus pressure up into high pressure ranges of several 1,000 bar,

[0024] with decreasing average grain size, the pressure rise gradient of the propellant reaction is markedly reduced in spite of an increase in surface area, and

[0025] in contrast to the dependencies of the Pmax value on the grain size of hexogen, in case of the use of beta-octogen the maximum pressure surprisingly decreases with decreasing grain size.

[0026] A preferred active binder along the lines of the present invention is polynitrophenylene. In case less temperature-sensitive propellants are not disadvantageous it is, however, also possible to employ nitrocellulose as the active binder.

[0027] A preferred use of beta-octogen according to the present invention resides in using same in propellants which contain e-octogen, polynitrophenylene, hexogen (cyclonite, RDX), guanidine nitrate, hexanitrodiphenylamine, dipicrylsulfone, hexanitrostilbene and/or tetranitrodibenzo-1,3a-4,6a-tetraazapentalene as additional explosives.

[0028] Propellants according to this invention contain, besides active binders, also inactive binders. Among these are understood to be, in particular, also those based on a synthetic resin. Since the use of binders results, besides in a reduction of detonation sensitivity, also in a strengthening of crystalline powder, good tackifying properties as well as a good dimensional stability within a temperature range from −40° C. to +70° C. are desirable. The binders should moreover be free of halogen and should yield a low quantity of solid combustion products.

[0029] Preferred binders are those based on polyurethanes, polymethyl acrylates, polyvinyl acetates, silicone, and polyvinyl alcohols, especially partially or completely acetalized polyvinyl alcohols with C₁- to C₅-aldehydes, obtainable per se in commerce. In this connection, polyvinyl butyral resin is of special significance; this compound is commercially obtainable. The type and amount of binders depend on the desired usage, especially for regulating the internal ballistics after shaping. Preferably, the amount of active and inert binders amounts, in each case independently of each other, to 5-15% by weight, based on the propellant.

[0030] In addition to the above-mentioned ingredients, the propellants obtainable with of this invention can also contain conventional plasticizers, lubricants and/or stabilizers known in the art.

[0031] Such propellants can be utilized in loose or press-molded form. According to this invention, propellants in pressed form are utilized with preference.

[0032] The quantity of the multimodal beta-octogen in propellants of explosives, inert binders and active hinders is not critical.

[0033] Along the lines of the present invention, the multi-modal beta-octogen can be used without additional after-treatment in the above-cited propellants. A special embodiment of the present invention, however, resides in encasing the crystals of this beta-octogen with thermoplastic polymers prior to use in the aforementioned propellants.

[0034] Although it is intended in accordance with the present invention to utilize multimodal beta-octogen having an average grain size of less than 50 μm, it is especially preferred that at least 95% by weight of the beta-octogen grains also exhibit a grain size of less than 100 μm.

[0035] The invention will be described in greater detail by the following examples and comparative examples.

EXAMPLES 1-4/COMPARATIVE EXAMPLE

[0036] In a propellant produced according to Example 1 of DE 2,753,555 C1, consisting of the amounts of α-octogen as the energy carrier, inert binder and active binder recited therein, the proportion of energy carrier was, in part, replaced by beta-octogen having an average grain size of 23 μm. (Example 1 of DE 2,753,555 C1 is repeated herein). The propellant was processed into cartridges. As compared with the comparative example, no change was made in the heat of explosion of the propellant or in its mass or in the ignition system of the cartridge. The firing results were determined by means of a manometer. The mass of propellant charge powder was in each case 1.61 g. The thus-obtained data is set forth in Table I.

[0037] Example 1 of DE 2,753,555 C1

[0038] In a masticator, a mixture of 32 parts by volume of ethyl acetate, 4 parts by volume of toluene and 4 parts by volume of n-butyl acetate was added to a mixture, mixed by screening or by means of a tumbler mixer in the dry state, of 70 parts by weight of α-octogen, 6.5 parts by weight of polyvinyl n-butyral, 4.9 parts by weight of polynitropolyphenyl produced by Ullmann reaction from styphnic acid dichloride and pulverized copper in nitrobenzene at 180° C. and 10.6 parts by weight of ammonium hydrogen carbonate as the filler; the mixture was kneaded for 30 minutes. Then the mixture was extruded in a cylindrical tool having a diameter of 70 mm with 42 holes of 1 mm in diameter each, and the ropes were cut into granules having a length of <1 mm.

[0039] Respectively, 0.998 g of this granulated material was weighed and introduced into molds and press-molded under a pressure of 1.8 t/cm² into propellant half shells. The latter, after driving out the filler (4 hours at 100° C.), exhibited an impact resistance of 1.91 N/cm.

[0040] Expressed in weight percent, this means, for the substances remaining in the propellant (ammonium hydrogen carbonate serves for producing porosity and is driven out): 86% by weight of α-octogen, 8% by weight of PVB, and 6% by weight of PNP. Of these 86% by weight of α-octogen, 5, 10, 15 and, respectively, 20% by weight are replaced, in Examples 1-4, by fine-grained β-octogen. In Examples 5-9, α-octogen has been completely replaced by β-octogen. TABLE I Example No. Comp. Ex. 1 2 3 4 Proportion of beta- 0 5 10 15 20 octogen (% by weight) Firing results: Max. pressure* (bar) 4532 4626 4607 4527 4630 Standard deviation (bar) 187 173 226 181 189 Firing time (millisec.) 4.76 4.94 4.91 4.78 4.87 Projectile velocity 954 948 947 943 946 after 5 m (m/s) Standard deviation (m/s) 6.1 6.1 6.6 6.6 6.0

[0041] In spite of an increase in the beta-octogen proportion, no significant difference in internal ballistics can be noted when comparing Examples 1 to 4 with the comparative example.

[0042] Table II, set out below, indicates the grain size spectrum or distribution of beta-octogen having an average grain size of 23 μm. TABLE II Grain Size Distribution, beta-Octogen having Average Grain Size 23 μm <10  6.5% by weight 10-20 μm 46.5% by weight 20-30 μm 27.0% by weight 30-40 μm 10.0% by weight 40-50 μm  4.0% by weight 50-60 μm  2.5% by weight 60-80 μm  3.5% by weight  100% by weight

EXAMPLES 5 AND 6

[0043] A propellant analogous to Examples 1-4, containing exclusively beta-octogen as the energy carrier, as well as inert and active binders, was processed to cartridges. The composition of the propellants from Examples 5 and 6 was identical except for the difference of the octogen having average grain sizes of 23 and 9 μm, respectively. The 9 μm average grain size has an analogous grain size distribution as the octogen with an average grain size of 23 μm. The production of the cartridges, their mass of propellants, dimensions, and ignition systems were identical.

[0044] An investigation was made of the pressure/time curves during ignition and deflagration of the cartridges in a ballistic bomb, the internal geometry of which was similar to that of the cartridge chamber in the firearm system. The following Table III includes the thus-obtained data. TABLE III Example No. 5 6 Average grain size 23 9 beta-octogen (μm) Firing result in a measuring bomb Maximum pressure (bar) 4325 4116 Standard deviation (bar) 33 65 Firing time to maximum pressure 1.759 1.781 (millisec.) Standard deviation (millisec.) 0.126 0.174 Pressure rise gradient (bar/millisec.) % p_(max) 10-90 4153 3937 20-80 5022 4871 30-70 6382 5795

[0045] The results are averaged values from 10 firings.

[0046] The firing result clearly shows that, in spite of the same energy content, the pressure rise gradient when using an average beta-octogen grain of 9 μm lies markedly below that for 23 μm.

EXAMPLES 7 AND 8

[0047] The same propellants as described in Examples 5 and 6 were press-molded to cylinders having the dimensions of about 4.5×4.2 mm. These were ignited in a ballistic bomb with a charging density of 290 kg/m³ with a primer pellet T 15 and a secondary charge of 4 10⁻⁴ kg nitrocellulose (13.2% nitrogen). The subsequently listed Table IV includes the thus-obtained data. TABLE IV Example No. 7 8 Average grain size 23  9 beta-octogen (μm) Firing results in a ballis- 21 50 −30 21 50 −30 tic bomb at ° C. Maximum pressure (bar) 3471 3489 2991 3450 3488 2916 Standard deviation (bar) 9 8 80 19 43 86 Pressure rise gradient (bar/millisec.), average values for % p_(max) 10-90 1873 2167 1550 1596 1788 1281 40-60 4799 5338 3382 3886 4268 2728

[0048] It is clear from Examples 7 and 8 that, in spite of enlargement of the surface area of the fine octogen grain of 9 μm as compared with a grain of 23 μm, the maximum pressure does not differ in the temperature range, and the pressure rise gradient is lower with fine β-octogen.

EXAMPLE 9

[0049] In correspondence with Examples 5 and 6, a propellant was mixed with the same proportion of energy carrier, but in this case of beta-octogen having an average grain size of 6 μm, inert binders, and active binders in the same composition, and cartridges having an identical ignition system were produced from this mixture in accordance with the same method.

[0050] The results of 30 firings, in the same manometer as utilized for Examples 1-4 and the comparative example, with a propellant powder mass of 1.64 g, are set out in the following Table V containing the averaged results: TABLE V Maximum pressure (bar) 2570 Standard deviation (bar) 556 Firing time (millisec.) 7.2 Projectile velocity (m/s) 766

[0051] A comparison with the results of Examples 1 through 4 and the comparative example shows a drastic reduction of the linear burning rate and of the maximum pressure.

[0052] In addition to the predominant influence exerted by the fine-grained β-octogen having an average grain size of 6 μm, other contributing factors are reduction of the heat of explosion from 3996 to 3541 J/g, as well as the reduction of porosity of the solid propellant powder articles. A comparison of the differences in porosity shows a pressure reduction by about 500 bar, with an otherwise identical composition of the propellant and under identical manufacturing and firing conditions.

[0053] It will be understood that the propellant of this invention comprises 70 to 95 wt. % explosive and 5 to 30 wt. % binder. The explosive is composed of 5 to 100 wt. % of beta-octogen and 0 to 95 wt. % of the other additional explosives heretofore described. 

We claim:
 1. A method of producing a propellant which comprises admixing of β-octogen having a multimodal grain size distribution and an average grain size of less than 50 μm as explosive in a propellant composition containing inert binders, and active binders.
 2. The method according to claim 1 , wherein the propellant also contains α-octogen, polynitrophenylene, hexogen, guanidine nitrate, hexanitrodiphenylamine, dipicrylsulfone, hexanitrostilbene and/or tetranitrodibenzo-1,3a-4,6a-tetraazapentalene as other additional explosive.
 3. The method according to claim 1 or 2 , wherein the inert binders comprise polyurethanes, polymethacrylates, polyvinyl acetates, silicones, and polyvinyl alcohols, especially partially or completely acetalized polyvinyl alcohols with C₁-C₅ aldehydes.
 4. The method according to claim 1 or 2 , wherein the active binder comprises polynitrophenylene.
 5. The method according to claim 1 or 2 , wherein at least 95% by weight of the β-octogen exhibits a grain size of less than 100 μm.
 6. A method according to claim 1 , wherein the propellant is prepared in a pressed form by appropriate compacting of the resultant admixture.
 7. A method according to claim 1 , wherein the β-octogen comprises crystals of β-octogen encased by thermoplastic polymers.
 8. A propellant which comprises grains of β-octogen as an explosive admixed with an active binder and an inert binder, said β-octogen having a multimodal grain size distribution and an average grain size of less than 50 μm.
 9. The propellant according to claim 8 , wherein the propellant also contains α-octogen, polynitrophenylene, hexogen, guanidine nitrate, hexanitrodiphenylamine, dipicrylsulfone, hexanitrostilbene and/or tetranitrodibenzo-1,3a-4,6a-tetraazapentalene as other additional explosive.
 10. The propellant according to claim 8 or 9 , wherein the inert binders comprise polyurethanes, polymethacrylates, polyvinyl acetates, silicones, and polyvinyl alcohols, especially partially or completely acetalized polyvinyl alcohols with C₁-C₅ aldehydes.
 11. The propellant according to claim 8 or 9 , wherein the active binder comprises polynitrophenylene.
 12. The propellant according to claim 8 or 9 , wherein at least 95% by weight of the β-octogen exhibits a grain size of less than 100 μm.
 13. The propellant according to claim 8 , wherein the propellant is prepared in a pressed form by appropriate compacting of the resultant admixture.
 14. The propellant according to claim 8 , wherein the β-octogen comprises crystals of β-octogen encased by thermoplastic polymers.
 15. The propellant according to claim 9 , wherein the propellant contains 70 to 95 wt. % of explosive and 5 to 30 wt. % of binder, the explosive comprising 5 to 100 wt. % of β-octogen and 0 to 95 wt. % of the other additional explosive. 